Abstract

The Beck Spring Dolomite is a platform carbonate sequence dominated by cryptalgal laminites with some stromatolites and grainstones and is inferred to have been deposited in shallow subtidal to intertidal environments. Cavity structures, where the cements occur, include birdseyes, laminoid fenestrae, stromatactis, sheet cracks, and neptunian dikes. Four types of synsedimentary-early diagnetic fibrous dolomite are distinguished. Type A forms isopachous fringes consisting of length-slow acicular to columnar pseudopleochroic crystals with fine, substrate-parallel color banding. The dolomite is mostly nonluminescent with some bright blotches, and inclusion trains, define acicular crystallites within the columnar crystals. Type B consists of length-slow columnar crystals with distinctive bladed relic structures, defined by inclusions and nonluminescence, surrounded by inclusion-free and brightly luminescent dolomite. Type C consists of bladed to wedge-shape pseudopleochroic crystals with undulose extinction (mostly fascicular optic); inclusion trains define acicular crystallites within crystals, and forms mammillated crusts and botryoids. Type D fibrous dolomite forms thin layers of acicular crystals, with a distinctive geometry of the cement layers thickening into the corners of cavities. Types B and C contain relic structures indicating replacement, but types A and D have fabrics which could be primary. The depositional components (pisolites and micrite) and some fibrous dolomites have oxygen isotopic values around -2 per mil versus PDB, compatible with a marine origin. However, Na (around 220 ppm) and Sr (around 65 ppm) contents are very low, suggesting precipitation from dilute waters. Meteoric-marine mixing-zone dolomitization of CaCO 3 sediments and early cements. Or primary marine precipitation of poorly ordered calcian dolomite and early diagenetic stabilization to stoichiometric dolomite. The dolomite spar has very negative values of delta 18 O (-9.6 per mil versus PBD) and is more depleted in Na (34 ppm) and Sr (47 ppm), reflecting precipitation from meteoric waters of very low ionic strength during burial diagenesis. A replacement origin for the dolomite of the depositional components and fibrous cements can be reconciled with the lack of obliterative replacement fabrics invoking an original high-Mg calcite mineralogy.--Modified journal abstract.

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